1. Field of the Invention
The present invention relates to superconducting oxide materials and processes for preparing the same. More particularly, it relates to superconducting oxide materials containing Tl and/or Bi having high superconducting critical temperature (Tc) and high superconducting critical current density (Jc), processes for preparing the same and their applications as wires or magnets.
2. Description of the Related Art
Discovery of new oxide type superconductor by Bednorz and Muller revealed the possibility of high temperature superconductors (Z. Phys. B64, 1986 p 189) and study to find much higher Tc superconductors have been continued.
C. W. Chu et al. reported another superconducting material of so-called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of about 90 K. (Physical Review letters, Vol. 58, No. 9, p 908).
Maeda et al reported the other type new superconducting compound oxide of Bi-Sr-Ca-Cu-O system which show the critical temperature of more than 100 K. (Japanese Journal of Applied Physics. Vol. 27, No. 2, p 1209 to 1210). Tl-Ba-Ca-Cu-O system compound oxide reported by Hermann et al. in Appl. Phys. Lett. 52 (20) p 1738 is also a high Tc superconductor of about 120 K. The Tl-Ba-Ca-Cu-O system and Bi-Sr-Ca-Cu-O system oxide superconductors show very high critical temperatures which are sufficiently higher than boiling point of liquid nitrogen of 77 K. and hence are expected to be applicable to actual uses.
However, the critical current density (Jc) of these systems at 77 K., drops sharply under higher magnetic field of more than 1 T. In fact, the Bi-Sr-Ca-Cu-O system oxide is a mixture of a high Tc phase of 100 K. and a low Tc phase of 80 K. and it is difficult to separate them. The Tl-Ba-Ca-Cu-O system compound oxide also is composed of a plurality of different phases such as Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x and Tl.sub.2 Ca.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y and is extremely difficult to isolate the high Tc phase alone. In the case of Bi-Sr-Ca-Cu-O system oxide, it was proposed to add Pb to this system in order to obtain nearly 90% single phase of Bi-Pb-Sr-Ca-Cu-O system having a critical temperature of higher than 100 K. However, in order to obtain this Bi-Pb-Sr-Ca-Cu-O system, the material powder must be subjected to very time consuming sintering operation. Still more, the resulting sintered oxides show anisotropy in crystal structure and exhibit a very low critical current density under strong magnetic field.
Tl-Sr-Ca-Cu-O system compound is also reported. This system shows relatively lower critical temperature of 75 K.
An object of the present invention is to overcome the problems of the prior art and to provide improved superconducting materials which exhibit higher critical current density under strong external magnetic fields as well as high critical temperature, processes for preparing the same and their applications as superconducting wire and magnets.
Another object of the present invention is to provide a process for producing single crystals of these superconducting compound oxides.